This invention relates generally to fiber amplifiers for WDM or DWDM systems and more particularly to improvements in gain flatness in such systems and designed for their future upgradability in power output and signal channel capability.
Wavelength division multiplexing (WDM) or dense WDM (DWDM) communication systems increase the transmission capacity of an optical fiber in a communication system by combining several optical signals with different wavelengths. To render these systems practical, optical amplifiers must meet tight requirements for providing gain uniformity over all channel signal wavelengths. Since the intrinsic gain spectrum of a rare earth doped fiber amplifier is highly non-uniform, some method of flattening the gain profile of the amplifier must be implemented. The gain profile of the fiber amplifier, however, is complicated by other factors as well. The gain profile of a rare earth doped optical fiber amplifier OFA), such as an erbium doped fiber amplifier (EDFA), is determined by the average inversion level of the erbium ions in the erbium doped fiber. This inversion level is a function of the power level of the signal or signals to be amplified and the applied power levels of the pump sources. If the signal power is sufficiently lower than the applied pump power, the fiber will maintain close to 100% inversion and the signal gain and the amplifier gain profile will not appreciably change with changes in input signal powers. However, as signal powers increase, the signal gain in the amplifier becomes limited by the availability of pump power for the fiber amplifier, i.e., output signal power is, of course, limited by the available pump power and the signal input power levels. OFA""s usually operate with their gain fibers saturated because this provides for optimum pump to signal power conversion. When the gain of the amplifier is xe2x80x9csaturatedxe2x80x9d, the inversion level, amplifier gain, and the gain profile of the amplifier all become strongly dependent on the ratio of signal to pump powers. The standard manner of maintaining constant gain flatness with changes in input power levels is to actively change the pump power to all OFA stages of the system in order to maintain constant signal to pump power ratios. There is a need, therefore, to provide a WDM amplifier system with flexibility to provide a commercially viable way to provide for maintenance of constant pump to signal ratios without redesigning the amplifier system. It is good to accomplish this by increasing the pump power but this must be done without saturating the input amplifier stage of the system, also referred to as the pre-amplifier; otherwise, the noise figure is materially affected.
While much progress has been made in achieving wide bandwidth gain flattened OFA systems, such as the system illustrated in Masuda et al., xe2x80x9cWideband, Gain-Flattened, Erbium-Doped Fibre Amplifier With 3 dB Bandwidths of  greater than 50 nmxe2x80x9d, Electronic Letters. Vol. 33(12), pp. 1070-1072, Jun. 5, 1997, the achievement of higher output powers required to make these systems more practical have not achieved much attention as well as optimization and adjustability of the gain flattening characteristics of the system.
In providing additional pump power, it is desired that the OFA system lend itself to be easily adaptability to power upgradability, allowing customers to purchase a lower capacity amplifier system and, later, upgrade the system to increase system capacity with increasing communication demands while providing the same output gain per channel and no significant change in system noise figure.
Therefore, it is an object of this invention to provide a multistage fiber amplifier system that is field upgradable in pump power and channel capacity.
It is another object of this invention to provide a multistage fiber amplifier system that is upgradable to higher output power levels and increase in channel capacity while maintaining uniform signal output per channel.
A further object of this invention to provide an upgradable multistage fiber amplifier system that allows a wide variation in possible external gain configurations while employing the same interstage gain equalizer continually maintaining uniform amplifier gain flattening in spite of changes to the pump power and signal channel capacity to the base multistage amplifier system.
According to this invention, a multistage fiber amplifier system for primary use in optical communication links comprises a pre-amp input optical fiber amplifier (OFA) stage and a power output optical fiber amplifier (OFA) stage and includes a gain flattening filter (GFF) is field upgradable in pump power and channel capacity while maintaining substantially the same signal power to pump power ratio when upgraded so that the characteristics of the gain flattening filter remain applicable to the upgraded system. An important factor in achieving such upgradability is that in the case of either the basic system configuration or in the upgraded configuration, a high inversion state, i.e., the inversion in the input stage of the system is maintained as high as practical or close to 100%. This can be accomplished more easily by fabricating a short fiber input stage with excess pump power in the input stage provided to bypass the gain flattening filter and launched into the output power amplifier stage. Also, the pump power is provided via an optical split coupler to provide pump power to both the input and output OFA stages. Upon system upgrade with added pump power, the additional pump power can be provided through an input connector to the optical coupler. With pump power upgrade, a proportionately amount of increase in pump power will be provided to the input and output OFA stages.
Also, disclosed are ways to enhance the gain flattening filter attenuation profile as well as ways to adjust and control the uniformity of the external gain tilt and the external gain level of the amplified channel signals in mass production of the multistage amplifier systems.
These and other features of the invention are expressed in further detail in the description and discussion of the redundant pumping scheme and its application to a lightwave communication system as illustrated in the accompanying drawings.